The present invention relates to a transmission for a vehicle such as an automobile, and more particularly relates to an improvement in an automatic transmission for the purpose of reducing its axial length.
A conventional automatic transmission for a vehicle commonly includes a gear transmission mechanism which is selected between various speed stages by selective engagement of various friction engaging devices, including clutches and brakes. Such friction engaging devices generally include a plurality of plate shaped friction engaging elements, which are parallel to one another and mutually oppose one another. One set of these friction plate elements rotates about its axis of symmetry relative to the other set of these friction plate elements, and a hydraulic piston device is arranged so that when it is supplied with hydraulic pressure it squeezes together axially the first and second sets of friction plate elements, thus engaging them together and preventing relative rotation between them. Thus, according to selective engagement of such friction engaging devices, various speed stages are selected for the transmission. These friction engaging devices can either be clutches, which are defined as selectively engagable devices which couple together two rotating members which may be rotating at different speeds, or brakes, which may be defined as selectively engagable devices which couple a rotating member to a fixed member such as the transmission housing.
Conventional return devices for such hydraulic piston devices have included a spring which biases the piston in the direction to reduce the size of the pressure chamber which actuates it. Conventionally and commonly, the piston of the hydraulic piston device has been formed with a hollow facing in the direction in which it is impelled by the pressure chamber, and the spring has been disposed within this hollow space, with its one end bearing against the piston and its other end bearing against a member which does not move axially, so that the spring opposes the motion of the piston.
Nowadays an increasing number of vehicles such as automobiles are being constructed in the front engine front wheel drive configuration. In such a construction, which typically uses a transverse engine, the axis of an automatic transmission generally runs parallel to the front axle of the vehicle. Accordingly, it becomes very important to reduce the axial dimension of the transmission as a whole as much as possible. This becomes particularly important when the automatic transmission is coupled with a subtransmission.
For reducing the total axial dimension of the automatic transmission, it is very desirable for components of the transmission to be overlapped axially as much as possible, and particularly it is very desirable that components of the friction engaging devices in the transmission should be arranged to overlap axially with parts of the gear transmission mechanisms in the automatic transmission, such as the gear wheels and planetary mechanisms. Such overlapping, however, means that the maximum possible radial extent of the hydraulic piston devices becomes greatly reduced. Accordingly, the above outlined construction for housing the return spring for the hydraulic piston device within a hollow chamber facing axially along the direction of motion of the piston becomes impracticable, because there is not enough radial space available on the piston for such a construction.